鉄骨 and 鉄筋: steel beams and reinforcing bars
Compare 鉄骨 used in a frame or lifted by a crane with 鉄筋 tied into a frame or visible inside a wall. Follow which member is described as thick or rusty.
Steel members named 鉄骨
鉄骨 (てっこつ) means a steel frame or beam. 「建物の骨組みに太い鉄骨が使われている。」 means “Thick steel beams are used in the building's frame.” 太い modifies the steel beams used in the frame.
「作業員が鉄骨をクレーンで持ち上げた。」 means “A worker lifted the steel beam with a crane.” The action is lifting the beam with a crane. Lifting alone does not state that installation is complete.
Reinforcing bars named 鉄筋
鉄筋 (てっきん) means a reinforcing bar or rebar. 「職人が鉄筋を組んで建物の骨組みを作った。」 means “The craftsperson tied reinforcing bars together to form the building's frame.” 鉄筋 also appears in a frame-building example.
「古い壁の中にさびた鉄筋が見えた。」 means “Rusty rebar was visible inside the old wall.” さびた describes the visible rebar as rusty. The observed object is the rebar inside the old wall.
Read the member and its modifier together
鉄骨 and 鉄筋 name different members, but both examples connect them with a building’s 骨組み. The presence of the word frame does not automatically select just one of these names.
太い鉄骨 names thick steel beams, while さびた鉄筋 names rusty rebar. Distinguish the member being lifted from the member visible inside the wall.
What do 太い鉄骨 and さびた鉄筋 name?
Show the answer and why
The first names thick steel beams used in the frame; the second names rusty rebar visible inside the old wall. Both have frame-related examples, but their names and modifiers remain distinct.